There is usually a pressure reading point on the side, for plugging external pressure gauges into. All that will tell you is the line pressure, which changes dependant on many variables, including temperature. Does the fluid still go through the radiator, or is it just going to the front oil cooler?
Do you know what box it is? Patrol/300z boxes can be built for 1200hp. If it's a stock box built, you are probably near its limit.